The dental laboratory technician is familiar with a variety of methods for preparing a metal substrate prior to porcelain application in the construction of porcelain-to-metal dental crowns and bridges. Generally, the construction of such a porcelain-fused-to-metal or ceramo-metal prosthesis begins with production of a metal casting or substrate. The surface of the metal substrate must be prepared to receive and bond to a porcelain overlay.
The most commonly used materials and method for preparing the metal substrate involves the use of mounted abrasive stones that are attached to a rotary handpiece. When the handpiece is activated the operator uses the rotating abrasive stones to grind the surface of the metal substrate to reduce the thickness of the metal substrate by 0.2 mm to 0.3 mm. The typical handpiece used for this procedure is operated a speed of about 45,000 RPM.
Use of mounted abrasive stones typically requires a great deal of strength in order to apply sufficient pressure to the stones to grind the metal substrate into the desired shape. The amount of pressure and friction created upon the metal substrate can result in significant heat build up at the stone metal interface and may pose a heat hazard to the fingers of the operator. The procedure is typically quite time-consuming as well as somewhat hazardous.
Mounted abrasive stones are constructed of a mixture of various sized stone particles and a cement-like binding substance. The mixture is shaped and mounted upon a mandrel and allowed to harden. It is believed that when a mounted abrasive stone is used to abrade a metal surface, the stone begins to break down. The loosened ingredients in the mounted abrasive stones include particles of abrasive materials mixed with cement and other substances at the stone-metal interface. If these materials are not adequately cleaned from the metal substrate, it can contaminate the porcelain-to-metal interface and compromise the strength of the porcelain-to-metal bond. It is also believed that the metal surface created by abrasive mounted stones may exhibit porosities that cannot be determined by visual inspection. Since the life of the crown is directly related to the bond between the metal substrate and porcelain overlay or veneer, it can be seen that surface preparation and the resulting bond is critical.
A further disadvantage of using abrasive mounted stones involves the change in shape of the stone due to wear over short time, thereby compromising consistency of the finished surface of the metal substrate.
Another tool, which is sometimes used to finish a metal dental substrate, includes a conventional carbide crosscut bur. Although used to a lesser degree than stones, these conventional burs are known to leave in its wake a very smooth surface/finish. It is still believed by some technicians in the art that a smooth satin finish yields a proper surface on which to bond the porcelain. It is believed that these burs leave a smooth or clean surface due to the fact that dentates on adjacent blades or flutes of the bur are oriented to have an amount of overlap that assists in the removal of unwanted material on the entire surface being prepared. However, the conventional crosscut bur has a tendency at a high rotational speed to skip or bounce off the surface to be prepared, which can make it difficult to control and fatiguing to hold and furthermore, necessitates that the bur is used at a relatively low rotational speed, such as about 45,000 RPM.
A demand exists therefore for an improved device and method for preparing metal substrate surfaces for receiving a porcelain veneer. The present invention satisfies the demand.